PROBABILISTIC RISK BASED APPROACH FOR PERFORMING AN ONSTREAM HIGH TEMPERATURE HYDROGEN ATTACK INSPECTION

摘要

A risk based methodology is proposed for the assessment of HTHA (high temperature hydrogen attack). This approach permits the ranking of probability in a semi-quantitative fashion for establishing inspection requirements. The assessment methodology relates the Pv parameter to the operating limits/curves in API 941, where Pv correlates temperature and hydrogen partial to the time for incipient attack1. This, in turn, is correlated to the probability of failure as described on a risk matrix. The results are then used to establish inspection frequency, extent, and priority. The approach outlined here was applied to a C-0.5Mo pressure vessel operating above the carbon steel API 941 curve2. Due to the variable performance of C-0.5Mo steel in hydrogen service there have been a number of unfavorable service experiences with this material, resulting in the removal of the C-0.5Mo curve from API 941. Risk analysis of the pressure vessel evaluated in this study indicated the need for inspection. The inspection for HTHA is traditionally performed when equipment is out of service. However, an on-stream inspection has the advantage of identifying damage before the equipment is shut down and thereby providing adequate lead-time to prepare for replacement or repair. An on-stream inspection program was developed, qualified, and successfully field deployed for the aforementioned vessel, which operates up to 750oF (400℃). The inspection approach relied on the ultrasonic backscatter spectrum analysis method for the base metal inspection and time of flight diffraction – ultrasonic testing (TOFD UT) for weld inspection. This paper will describe the approach and results from the on-stream vessel inspection and a subsequent off stream inspection, which validated the on-stream results.